The present invention relates to a method for treating mammals having acquired immunodeficiency syndrome (AIDS) or AIDS-related complex or who are infected with other retroviruses, which comprises the administration of a therapeutically effective amount of a purine or pyrimidine acyclonucleoside. Certain compounds of this class, such as the guanine derivatives, have been found to have activity against the herpes class of viruses such as herpes simplex virus. See European patent application No. 82401571.3. However, it has not been previously shown that these compounds are effective against retroviruses such as human immunodeficiency virus (HIV), the cause of acquired immuno-deficiency syndrome (AIDS) and AIDS related complex.